Automatic top-loading weigh apparatus with electronic measuring and recording circuit

ABSTRACT

A top-loading weigh apparatus has a stabilized balance beam mechanism with the load being vertically applied through a horizontal balance arm on a pair of spaced pivots, and a differential capacitance is developed by sensing the horizontal displacement of a stabilized beam isolated from the weighing area. The capacitance developed in response to beam displacement is converted into a variable current output and applied as a restoring force proportional to but in opposition to the unbalancing force in order to quickly and accurately return the beam mechanism to its null position. A voltage is developed in direct proportion to the variable current output required to restore the beam to its null position and which is translated for recording and visual display of the applied weight.

United States Patent 3,464,508 9/1969 Engleetal.

[72] Inventors Charles B. Blethen 177/210 X Green Mountain; 3,368,6372/1968 Green et a1. 177/21 1 I g gg Longmont both Colo PrimaryExaminerRichard B. Wilkinson [21] p 9 Assistant Examiner-George H.Miller,.lr.

[22] Flled 1969 Attorney-Reilly and Lewis [45] Patented Sept. 14, 1971[73] Assignee Wm. Ainsworth, Inc.

Denver, C010.

[54] AUTOMATIC TOP-LOADING WEIGH APPARATUS WITH ELECTRONIC MEASURING ANDABSTRACT: A top-loading weigh apparatus has a stabilized RECORDINGCIRCUIT balance beam mechanism with the load being vertically ap-10C|aims,6 Drawing Figs plied through a horizontal balance arm on a pairof spaeed pivots, and a differential capacitance 18 developed by sensing[52] [1.8. Ci 177/210, the horizomm displacement ofa stabilized beamisolated from 177/212 the weighing area. The capacitance developed inresponse to [51] Int. Cl 601g 3/14 beam displacement is converted into avariable current output [50] Fleld of Search 177/1, 200, and appned as arestoring force proportional to but in opposi 211 tion to theunbalancing force in order to quickly and accurate- [56] ReferencesCited 1y return the beam mechanism torts null position. A voltage 18developed in direct proportion to the variable current output UNITEDSTATES PATENTS required to restore the beam to its null position andwhich is 2,680,012 6/1954 Bozoian 177/210 X translated for recording andvisual display of the applied 2,734,736 2/1956 Payne 177/212 X weight.

J i |1si c DIFFERENTIAL SAIPLEQ How L osc. AMPLIFIER quadrature M1061 l41 a/ 88 a4 1 l6 7 LEVEL DETECTOR 87 5 l3 92 FREQUENCY COMPENSATION 2894) 15 POWER MP FIER A L1 VPATENTEDSEPIMQYI 3504525 SHEET 3 UF 3 FIG. 6

INVL'N'IURS CHARLES B. BLE THEN BY JAMES E. SMI TH ATTORNEY AUTOMATICTOP-LOADING WEIGI-I APPARATUS WITH ELECTRONIC MEASURING AND RECORDINGCIRCUIT This invention relates to a novel and improved weigh balancesystem and more particularly. relates to an automatic top-loading weighapparatus provided with means for electronically measuring and recordingstationary weights in an accurate, highly dependable manner.

It is customary practice to provide weight balance mechanism systemswherein displacement of a beam is sensed and converted to an electricalcontrol signal which in turn is converted and applied as a restoringforce to maintain the beam in its null position; and the amplitude ofthe signal developedmay be translated into a weight recording. There isa definite need, however, for a balance system, especially of thetop-loading type, which minimizes instrument errors and inaccuracies inmeasurement and recording, either introduced in the mechanical weighsystem or in the electronic sensing and signal generating circuit. It istherefore proposed, in accordance with the present invention, to devisea novel and improvedweigh balance system for electronically measuringand recording stationary weights which is highly sensitive, accurate,and greatly reduces instrument errors in measurement and recording, suchas, for example, errors resulting from the placement of weights atdifferent locations on the weigh pan.

It is another objective and feature of the present invention to providefor a noveland improved null balance system capable of achieving rapidrepetitive measurement and recording of stationary weights; and further,a balance system which will effect linear beam displacement underapplied loads to generate a controlsignal and resultant restoring forceproportional and in opposition to displacement of the beam in either apositive or negative direction from its null position.

It is .a further object of the present invention to provide in anautomatic weight-measuring and recording system for novel and improvedmonitor and control circuit means to capacitively sense displacement ofthe beam-either ina positive or negative directiornto differentiallymeasure such displacement and convert same .to a variable current outputwhich in turn is translated into-a restoring force to return the beam toits null position, said circuit being characterized by its ability torespond to and accurately measure sudden weight increases withoutoverloading of the circuit while being closely sensitive to minimalweight variations.

It is a still further object of the present invention to provide for acompact, highly sensitive, top-loadingweigh apparatus which willaccurately weigh and record stationary loads with very .high precisionand wherein the applied load is sensed through linear displacement of astabilized vertical beam isolated from the load.

The present invention contemplates a top-loading weigh apparatus in.which a load or unbalancing force is applied vertically to a stabilizedbalance mechanism-having a balance arm supported by a pair ofhorizontally spaced, oppositely directed, knife-edged pivots. A pair ofvariable capacitors disposed between the apparatus frame and a verticalleg forming a part of the balance mechanism serves to develop adifferential capacitance which can be accurately measured and convertedinto a variable current output. The output is translated and applied asa restoring force proportional but in oppositionto the unbalancing forceto instantaneously return the leg to its null position. A referencecircuit including a resistor simultaneously generates a voltage indirect proportion to the current required to restore the beam to itsnull position.

In the electronic measuring circuit, the variable capacitors areincorporated as a part of a capacitive bridge between an oscillatoremployed to drive or excite the bridge and differential amplifier, thelatter measuring the differential capacitance of the variable capacitorsin the bridge, and the phase and amplitude of the output from thedifferential amplifier are detected by a sampling circuit to determinethe direction and extent of movement of the beam leg. The samplingcircuit demodulates the output of the amplifier while eliminating anyunwarranted quadrature component from the output signal, and generates aDC voltage proportional to the differential capacitor. The voltageproduced is amplified and translated into a variable current output todrive a forcer coil in a direction returning the vertical beam leg toits null position.

The above and other objects, advantages and features of the presentinvention will become more readily understood and appreciated from aconsideration of the following detailed description of a preferred formthereof when taken together with the accompanying drawings, in which:

FIG. 1 is a block diagram of a preferred form of balance control systemincluding a schematic illustration of a top-loading weigh apparatus.

FIG. 2 is a front view of the preferred form of top-loading weighapparatus in accordance with the present invention.- 1

FIG. 3 is a top plan view thereof.

FIG. 4 is a side elevational view of the preferred form of top-loadingweigh apparatus.

FIG. 5 is a sectional view taken about lines 5-5 of FIG. 2; and

FIG. 6 is a circuit diagram of the preferred form of measuring andcontrol circuit.

Referring in more detail to the drawings, there is schematically shownin FIG. 1 a preferred form of weigh apparatus in accordance with thepresent invention wherein a top-loading balance unit 10 is broadlycomprised of a weigh pan 12 having a first vertical stirrup leg 13depending downwardly from a stirrup 14, including a bearing mount 14',the latter resting on a first knife edge pivot 15. The upwardly facingknife edge pivot 15 is .disposed in horizontally spaced relation to asecond, downwardly facing knife edge pivot 16 which rests on a bearingmount 17 supported by columns 18 risingupwardly from base frame 20. Thebase frame 20 is shown in FIGS. 2-5. In addition, a second vertical beamleg 22 depends downwardly in spaced relation to the knife edge pivot 16.A stabilizer in the form of a horizontal pivot rod 24 extends betweenthe legs 13 and 22 in order to form a parallelogram between the pivotrod, beam legs and spaced pivots whereby an applied load, such as, theload L in causing vertical displacement of the weigh pan 12 about theknife edge pivots I5 and 16 is accurately translated into horizontalsubstantially linear displacement of the legs 13 and 22. Thedisplacementresulting from the applied load is multiplied through the increasedlength of the beam legs relative to the moment arm of the weigh panabout the pivot 16 and is sensed through the beam leg 22 which isisolated from the load in the manner shown. It is noted that regardlessof where a load L is placed on the weigh pan 12 that the moment arm ofthe pan 12 is constant about the pivot 16 since all weight on the pan 12is applied to the knife edge of the pivot 15. Further, since the leg 13which depends from the stirrup 14 and the leg 22 are maintained parallelby the rod 24, the resultant force appliedat the knife edge of the pivot15 is always equal to the weight L on the weigh pan 12. Therefore, thebalance unit 10 operates to weigh a load L on the weigh pan 12.

Horizontal displacement of the balance beam mechanism is accuratelysensed through a sensor or probe, broadly designated at 26, which isarranged between an upright support member 27 on the frame and the lowerend of the leg 22. Electromagnetic actuating means represented at 28 issimilarly arranged between the lower end of the beam leg 22 and theframe support 27; and any displacement of the beam leg 22, as sensed bythe sensor 26, is converted to an electrical signal through a monitorand control circuit represented at 30 which generates signals in theform of a variable current output proportional to displacement of thebeam from its null position for application in the form of a restoringforce to the actuating means 28. Specifically, in the preferred form,the sensor circuit is defined by a capacitive bridge comprising a pairof fixed capacitors 31 and a pair of variable capacitors 32.

The variable capacitors are made up of sensor plate 33, supported onshaft 33 to follow movement of the beam leg 22,

and disposed on opposite sides of a pair of sensor plates 34 separatedby a phenolic insulator 34' and attached to an extension 27' of support27. Thus, the beam-mounted plates 33 are movable with the beam withrespect to the framemounted, or fixed, plates 34 so that movement of thebeam in one direction will be reflected by an increased capacitance inone of the capacitors and a decrease in the other; whereas, movement ofthe beam in the opposite, or negative, direction is conversely reflectedby an increased capacitance in the other variable capacitor and adecrease in the one capacitor. The relative increase or decrease incapacitance is differentially measured by the monitor and controlcircuit, converted to a variable output and is applied to the magneticfield of a forcer coil 35 which interacts with that of a permanentmagnet 36 in the actuating means 28. A beam-restoring force proportionalto the forcer coil current is therefore created to drive the beam backto its null position as indicated by the capacitor sensor plates 33 and34. The coil current required to maintain a null position on the beam isalso monitored through a reference circuit, including a referenceresistor 35' in series with the forcer coil 35, as shown in FIG. 6, inwhich the voltage across the reference circuit is proportional to thecoil current and hence proportional to the required beam restoringforce. This voltage may be conventionally converted to a digital numberthrough a digital volt meter for recording and visual display of theweight.

Considering in more detail the construction and arrangement of thebalance unit 10, as shown in FIGS. 2 to 5, the weigh pan 12 has adownwardly extending sleeve 42 mounted on spindle 43 projecting upwardlyfrom the stirrup 14, the latter having laterally spaced, downwardlydepending bearing mounts 14'. Each bearing mount 14 is conventionallymade up of downwardly facing, oppositely inclined surfaces 47 and 48,and flat metal bearing plates 49 are rollable on ball bearings50'suitably contained in a plastic insert, now shown, between thebearing plates and surfaces 47 and 48. The bearing mount assemblies asdescribed are self-centering on the upwardly projecting knife edge pivotcontained in a holder 52 which is affixed to one end of a balance arm54. The arm 54 also supports a downwardly directed holder 55 for thesecond knife edge pivot 16 to support the pivot in downwardly facingrelation to the front bearing mounts 17. The bearing mounts 17 areformed in a manner corresponding to that of the bearing mounts 14 andare supported on either side of a horizontal support 56 extendingbetween column legs 18. It will therefore be seen that the front knifeedge pivot 16 and bearing mount 17 are correspondingly formed to thoseof the rear pivot 15 and bearing mount 14' but in oppositely directedrelation to one another.

The weigh pan 12 extends rearwardly in horizontally offset relation tothe first pivot 15, and the rear leg 13 extends vertically fromconnected relation to the stirrup 14, inclines rearwardly and downwardlyand terminates in a lower vertically disposed leg portion 58 whichsupports a threadedly adjustable horizontal pivot 59 for the pivot rod24. In turn, the vertical beam leg 22 depends downwardly from theopposite end of the balance arm 54, and the leg 22 is provided with avertically adjustable pivot 60 disposed in a central opening therein.The horizontal pivot rod 24 has jewelled bearings at opposite ends 61for insertion in shallow concave recesses 62 in the horizontal andvertical pivots 59 and 60, respectively. The pivots 59 and 60 arethreadedly adjustable as illustrated to permit accurate vertical andhorizontal adjustment of the pivot rod between the legs 13 and 22 withthe pivot rod supporting the legs in spaced parallel relation to oneanother. Here the horizontal balance arm 54 together with the verticallegs 13 and 22 and pivot rod 24 form a parallelogram which is accuratelyadjustable through the pivot mounts on the legs to respond to variableloads placed on any part of the weigh pan without error. Moreover, ahorizontal limit bar 64 is adjustably mounted for extension between theframe 27, the front beam leg 22 and the rear stirrup leg 13 in order tolimit horizontal displacement of the legs 13 and 22 in response toapplication of a given weight thereby confining displacement of the legsto a nearly linear path and prevent swinging in a wide circular arc.

In order to limit vertical shifting of the balance mechanism when inuse, upper and lower connected plates 66 and 67 are arranged above andbeneath the bearing mounts and knife edge pivots between the column legsby connecting bolts 68. In order to shift the bearing mounts away fromthe knife edge pivots when not in use, an arrestment device includes atransverse horizontal shaft 70 extending between the column legs 18 andhaving a pair of cams 72 on either side of the beam leg 22. The earns72, when rotated by handle 71 will engage a pair of pins 73 to shift apair of associated arms 74 upwardly. The arms 74 have verticalextensions 75 with abutments 76 engageable with shoulders on the rearstirrup leg 13, and upwardly extending posts 77 are engageable with thelower plate 67 to simultaneously lift the legs 13 and 22 a distancesufficient to separate the bearing mounts from their respective knifeedge pivots. The posts 77 are spring-loaded normally to urge the arms 74downwardly against the arrestment cams 72. Thus, when the cams arereleased, the upper bearing amount 14 and knife edge pivot 16 asdescribed are free to return to centered relation against the lowerknife edge pivot 15 and bearing mount 17.

Referring in more detail to the monitor and control circuit 30 shown inFIG. 1, it will be observed that the fixed capacitors 31 of thecapacitive bridge circuit are arranged in parallel between an oscillator80, used to excite the sensor plates, and a differential amplifierrepresented at 82. The output of the differential amplifier is asinusoid with an amplitude that is proportional to the differencebetween the two variable capacitors 32; and the phase of thedifferential amplifier output will either lead or lag that of theoscillator 80 depending upon the relative magnitude of the variablecapacitors 32. A capacitor 83 and resistor 84 at the output of theamplifier serves to eliminate the DC portion of the amplifier outputinto a sample and hold circuit represented at 86. A level detectorcircuit 87, preferably defined by a Schmitt trigger, is driven by theoscillator 80 over line 88 and provides synchronous sample pulses foruse in the sample and hold circuit as hereinafter described. Generally,the circuit 86 functions as a multipurpose device that synchronouslydemodulates the output of the differential amplifier and eliminates theunwanted quadrature component from the output signal. From the circuit86, the output is a DC voltage the amplitude of which is proportionalonly to the difference between the variable capacitors 32 and thepolarity of which is dependent upon the relative magnitude of thevariable capacitors. The output from the sample and hold circuit isapplied over line 90 to an operational amplifier 91 which is frequencycompensated by network 92 to effect an extremely high DC gain with thenecessary lead-lag compensation to stabilize the balance beam mechanism.This is of particular importance in a balance control system of the typedescribed, since the beam position or displacement is measured to drivethe system forcer coil in returning the beam to its null position.

The output from the operational amplifier 91 is applied over line 93 toa power amplifier circuit 94. When the amplifier output is negative, theforcer coil 35 is driven in a negative direction to effect nullrestoration for small negative loads. When the amplifier output ispositive, it will develop a large coil current in the positive directionfor driving the forcer coil.

The preferred form of monitor and control circuit 30 is illustrated inmore detail in FIG. 6 wherein the variable capacitor terminals arerepresented at T1 and T2, the ground terminal at T3 and positive andnegative power input and output terminals at T4 and T5, respectively.The resistor-capacitor combination I00 and 101 at the input T4 as wellas the resistor-capacitor combination 102 and 103 at the output T5 serveto decouple the input power circuit. The oscillator circuit 80 includesa transistor 104 and associated resistorcapacitor components whichaffects only the system sample raised in the impedance of thecapacitance sensors. Utilizing a high input impedance differentialapproach, the capacitive sensor is essentially immune to oscillatorfrequency variations.

Field effects transistors 105 and 106 make up the input stage of thesystems AC differential amplifier. 82; transistor 107 and associatedcomponents provide a constant bias for the input transistors; and adifferential feedback is furnished by resistors 108 and 108" from theoutput transistors 109 and 110. The resistor-capacitor combination 112defines a DC block between the output of the differential amplifiercircuit and the input to the sample and hold transistor 114. In turn,the level detector 87 includes transistors 115 and 116 which aretriggered by the oscillator output and are differentiated prior todriving a transistor 118. The output of transistor 118 is normallynegative voltage with a limited pulse, on the order of l microsecond,going to ground and the pulses are synchronous with the output of thedifferential amplifier 87 and are phased to align with the peak outputsignal. Alignment is on the positive or negative peak, depending uponthe relative magnitude of the sensor capacitors. Thus transistor 114 isactivated during the synchronous sampling time; and once each periodwithin the constant of the oscillator, it transfers the peakdifferential amplifier output voltage onto the hold capacitor 120. As aresult, the DC voltage on capacitor 120 is equal to the peak AC outputvoltage of the differential amplifier which is only the capacitivecomponent of the differential amplifier output, and is positive ornegative depending upon the disposition of the balance beam with respectto its null position. As stated earlier, this DC voltage is the input tothe systems operational amplifier 91 which includes a resistor 122 andcapacitors 123, 124 for internal frequency compensation. Also acapacitor 125 is operative to roll off the high frequency gain of theamplifier and eliminate the voltage spikes that originate in the sampleand hold circuit. Resistors 126, 127 together with capacitors 128, 129define the lead-lag network 92 necessary for system stability.

The output of the operational amplifier drives the emitter follower pair130 and 131 with the resistor 132 acting as a current limiter foramplifier protection. Additionally, resistor 134 is a current limiterfor transistor 130. Resistor 135 eliminates the output deadband normallyassociated with the emitter follower circuits.

The circuitry described may be placed on printed circuit boards mountedin the section 140 of the base frame rear-' wardly of the balance beammechanism, as illustrated and in FIG. 3. In use, the base frame islevelled, for example, through levelling feet, not shown, at each cornerof the baseplate and with the aid of a spirit level 142 affixed to thebalance unit. All parts and components may be calibrated and adjusted ina conventional manner, for example, by employing an electronic testingunit, turning the power on and weighing calibrated weights at differentpositions on the pan. For the purpose of illustration and notlimitation, the preferred form of loading apparatus described isprimarily intended for use in weighing and recording of weights up to1,200 grams and achieves sensitivity on the order of 0.1 grams. Acounterweight 143 at the lower end of the beam leg 22, as well as thecapacitor probe 26 and the means 28 is manually adjustable incalibrating and setting the beam mechanism to its null position. When aload is applied to the weigh pan the force of displacement through thebeam leg is instantaneously sensed by the probe 26 and counteracted bythe forcer coil 35 to restore the beam to its null position.Accordingly, actual movement of the beam is negligible as a result ofthe rapid response and reaction by the sensing circuit to any changewhether positive or negative. An additional advantage and feature of thepresent invention is that the system is highly sensitive, yet is capableof sensing sudden weight increases without overloading the circuit.

It is therefore to be understood that the foregoing description is of apreferred embodiment of the present invention only, and it will beapparent to those skilled in the art that various changes andmodifications may be made therein without departing from the invention.

What is claimed is:

1. In an automatic weigh balance. having a balance beam wherein the loadapplied to the balance beam is measured by beam displacement from a nullposition, the improvement of:

variable capacitance means associated with said balance beam formeasuring the displacement of said balance beam by a change incapacitance;

an AC signal source for generating an AC reference signal,

said AC signal source being connected across said variable capacitancemeans to excite said capacitance means with said AC reference signal andgenerate an AC output representative of the change in the capacitance ofsaid capacitance means;

trigger means connected to said AC signal source for generating triggerpulses from said AC reference signal at the instant the AC output ofsaid capacitance means is only the capacitive component of said ACoutput, and

circuit means connected to said variable capacitance means andselectively triggered by said trigger pulses for generating a DC signalproportional to the magnitude of said AC output at the instants saidtrigger pulses are generated whereby to generate a DC signalproportional only to the capacitive component of said AC output of saidvariable capacitive means thereby to reject the unwanted quadraturecomponent of said AC output.

2. The invention recited in claim 1 wherein said variable capacitancemeans is a capacitive bridge having two legs of said bridge formed byvariable capacitors, one plate of each of said variable capacitors beingfixed relative to said balance beam and the other plate of each of saidvariable capacitors being connected to said balance beam for movementtherewith whereby the deflection of said balance beam from its nullposition is measured by the change in capacitance of said variablecapacitors, and including:

electrical null restoring means associated with said balance beam; and 7means including an operational amplifier connected between said circuitmeans and said null restoring means for driving said null restoringmeans in response to the DC signal generated by said circuit means anamount proportional to beam displacement whereby to return said beam toits null position.

3. In an automatic top-loading balance apparatus, the combinationcomprising a frame,

a balance beam mechanism including a stabilized vertical balance beamleg movable from a null position in response to variations in loadapplied to said balance beam mechanism,

electromagnetic actuating means between said frame and said balance beamincluding relatively movable field and core members with at least one ofsaid members having an excitable forcer coil,

a capacitive bridge including a pair of fixed capacitors and a pair ofvariable capacitors, each of said variable capacitors having aframe-mounted capacitor plate and a capacitor plate movable with saidbeam leg and so arranged as to sense the direction and extent ofmovement of said beam leg in a positive or negative direction,

oscillator means connected tosaid capacitive bridge for generating an ACreference signal and exciting said capacitive bridge therewith,

differential amplifier means for measuring the differential increase anddecrease in capacitance of said variable capacitors and for generating asinusoidal wave form proportional thereto,

a sampling circuit for sensing the amplitude and phase of each wave formgenerated by said differential amplifier, said sampling circuit beingselectively triggered by the AC reference signal to develop a variableDC output as a function only of the direction and extent of balance beammovement as measured by the changes in the capacitance of said variablecapacitors by sampling only the capacitive component of each wavegenerated by said differential amplifier means thereby to reject theunwanted quadrature component of each wave generated by saiddifferential amplifier means, and

means connected to said sampling circuit for receiving said DC outputand exciting the forcer coil of said electromagnetic actuating meanswith a DC signal proportional to beam displacement whereby to returnsaid balance beam to its null position 4. In an automatic top-loadingbalance apparatus according to claim 3, further including a leveldetector in said sampling circuit operative to generate sampling pulsessynchronous with the capacitive component of said sinusoidal wave formin response to the AC reference signal whereby to selectively triggersaid sampling circuit whenever one of said sampling pulses is generatedand develop said DC output.

5. In an automatic top-loading balance according to claim 4, whereinsaid means connected to said sampling circuit for receiving said DCoutput and exciting said forcer coil includes an operational amplifierhaving a frequency compensation network to drive said forcer coilnegatively or positively according to the negative or positive conditionof the variable DC output of said sampling circuit.

6. in an automatic weigh balance system, the combination comprising aframe,

a balance beam mounted on the frame for limited movement from a nullposition in response to variations in applied load,

electrical null restoring means associated with said balance beam,

sensor means for sensing the beam position in response to variations inapplied load including a pair of vehicle capacitors and an AC signalsource connected across said variable capacitors to apply an ACreference signal thereto, each of said variable capacitors having astationary capacitive sensor and a beam-mounted capacitive sensor soarranged as to differentially sense the direction and extent of balancebeam movement by changes in their respective capacitances,

differential amplifier means connected to said variable capacitors fordifferentially amplifying the AC signals generated across said variablecapacitors by said AC signal source and for generating a differential ACoutput in response to the direction and extent of balance beam movementas measured by the changes in the capacitances of said variablecapacitors,

circuit means for generating a DC signal as a function only of thecapacitive component of the differential AC output generated thereby toreject the unwanted quadrature component of the differential AC output,and

means connected to the circuit means for receiving the DC signalgenerated by said circuit means and energizing said null restoring meansan amount proportional to beam displacement whereby to return said beamto the null position.

7. In an automatic weigh balance system according to claim 1, whereinsaid AC signal source is an oscillator and further including acapacitive bridge, said bridge including said variable capacitors and apair of fixed capacitors, and said capacitance bridge being interposedbetween said oscillator and said differential amplifier means.

8. in an automatic weigh balance system according to claim 6 whereinsaid stationary capacitive sensors are mounted on said frame in adjacentrelation to one another and said beammounted capacitive sensors aredisposed on opposite sides of said stationary capacitive sensors.

9. In an automatic weigh balance system according to claim 6, saidelectrical null restoring means being defined by electromagneticactuating means having relatively movable field and core members with atleast one of said members having an excitable forcer coil.

10. In an automatic weigh balance system according to claim 9, furtherincluding a reference circuit having a resistor in series with the saidforcer coil to generate a voltage proportional to the signal applied tosaid forcer coil.

1. In an automatic weigh balance having a balance beam wherein the loadapplied to the balance beam is measured by beam displacement from a nullposition, the improvement of: variable capacitance means associated withsaid balance beam for measuring the displacement of said balance beam bya change in capacitance; an AC signal source for generating an ACreference signal, said AC signal source being connected across saidvariable capacitance means to excite said capacitance means with said ACreference signal and generate an AC output representative of the changein the capacitance of said capacitance means; trigger means connected tosaid AC signal source for generating trigger pulses from said ACreference signal at the instant the AC output of said capacitance meansis only the capacitive component of said AC output, and circuit meansconnected to said variable capacitance means and selectively triggeredby said trigger pulses for generating a DC signal proportional to themagnitude of said AC output at the instants said trigger pulses aregenerated whereby to generate a DC signal proportional onLy to thecapacitive component of said AC output of said variable capacitive meansthereby to reject the unwanted quadrature component of said AC output.2. The invention recited in claim 1 wherein said variable capacitancemeans is a capacitive bridge having two legs of said bridge formed byvariable capacitors, one plate of each of said variable capacitors beingfixed relative to said balance beam and the other plate of each of saidvariable capacitors being connected to said balance beam for movementtherewith whereby the deflection of said balance beam from its nullposition is measured by the change in capacitance of said variablecapacitors, and including: electrical null restoring means associatedwith said balance beam; and means including an operational amplifierconnected between said circuit means and said null restoring means fordriving said null restoring means in response to the DC signal generatedby said circuit means an amount proportional to beam displacementwhereby to return said beam to its null position.
 3. In an automatictop-loading balance apparatus, the combination comprising a frame, abalance beam mechanism including a stabilized vertical balance beam legmovable from a null position in response to variations in load appliedto said balance beam mechanism, electromagnetic actuating means betweensaid frame and said balance beam including relatively movable field andcore members with at least one of said members having an excitableforcer coil, a capacitive bridge including a pair of fixed capacitorsand a pair of variable capacitors, each of said variable capacitorshaving a frame-mounted capacitor plate and a capacitor plate movablewith said beam leg and so arranged as to sense the direction and extentof movement of said beam leg in a positive or negative direction,oscillator means connected to said capacitive bridge for generating anAC reference signal and exciting said capacitive bridge therewith,differential amplifier means for measuring the differential increase anddecrease in capacitance of said variable capacitors and for generating asinusoidal wave form proportional thereto, a sampling circuit forsensing the amplitude and phase of each wave form generated by saiddifferential amplifier, said sampling circuit being selectivelytriggered by the AC reference signal to develop a variable DC output asa function only of the direction and extent of balance beam movement asmeasured by the changes in the capacitance of said variable capacitorsby sampling only the capacitive component of each wave generated by saiddifferential amplifier means thereby to reject the unwanted quadraturecomponent of each wave generated by said differential amplifier means,and means connected to said sampling circuit for receiving said DCoutput and exciting the forcer coil of said electromagnetic actuatingmeans with a DC signal proportional to beam displacement whereby toreturn said balance beam to its null position.
 4. In an automatictop-loading balance apparatus according to claim 3, further including alevel detector in said sampling circuit operative to generate samplingpulses synchronous with the capacitive component of said sinusoidal waveform in response to the AC reference signal whereby to selectivelytrigger said sampling circuit whenever one of said sampling pulses isgenerated and develop said DC output.
 5. In an automatic top-loadingbalance according to claim 4, wherein said means connected to saidsampling circuit for receiving said DC output and exciting said forcercoil includes an operational amplifier having a frequency compensationnetwork to drive said forcer coil negatively or positively according tothe negative or positive condition of the variable DC output of saidsampling circuit.
 6. In an automatic weigh balance system, thecombination comprising a frame, a balance beam mounted on the frame forlimited mOvement from a null position in response to variations inapplied load, electrical null restoring means associated with saidbalance beam, sensor means for sensing the beam position in response tovariations in applied load including a pair of vehicle capacitors and anAC signal source connected across said variable capacitors to apply anAC reference signal thereto, each of said variable capacitors having astationary capacitive sensor and a beam-mounted capacitive sensor soarranged as to differentially sense the direction and extent of balancebeam movement by changes in their respective capacitances, differentialamplifier means connected to said variable capacitors for differentiallyamplifying the AC signals generated across said variable capacitors bysaid AC signal source and for generating a differential AC output inresponse to the direction and extent of balance beam movement asmeasured by the changes in the capacitances of said variable capacitors,circuit means for generating a DC signal as a function only of thecapacitive component of the differential AC output generated thereby toreject the unwanted quadrature component of the differential AC output,and means connected to the circuit means for receiving the DC signalgenerated by said circuit means and energizing said null restoring meansan amount proportional to beam displacement whereby to return said beamto the null position.
 7. In an automatic weigh balance system accordingto claim 1, wherein said AC signal source is an oscillator and furtherincluding a capacitive bridge, said bridge including said variablecapacitors and a pair of fixed capacitors, and said capacitance bridgebeing interposed between said oscillator and said differential amplifiermeans.
 8. In an automatic weigh balance system according to claim 6wherein said stationary capacitive sensors are mounted on said frame inadjacent relation to one another and said beam-mounted capacitivesensors are disposed on opposite sides of said stationary capacitivesensors.
 9. In an automatic weigh balance system according to claim 6,said electrical null restoring means being defined by electromagneticactuating means having relatively movable field and core members with atleast one of said members having an excitable forcer coil.
 10. In anautomatic weigh balance system according to claim 9, further including areference circuit having a resistor in series with the said forcer coilto generate a voltage proportional to the signal applied to said forcercoil.